The invention relates to a process for producing a cured silsesquioxane resin having high fracture toughness and strength without loss of elastic modulus. With more particularity the invention relates to a fractionation process for producing a cured silsequioxane resin having high fracture toughness and strength without the loss of elastic modulus.
Silsesquioxane resins have seen increased use in industrial applications in the automotive, aerospace, civilian and military and manufacturing industries. Silsequioxane resins exhibit excellent heat and fire resistant properties that are desirable for such applications. These properties make the silsesquioxane resins attractive for use in fiber-reinforced composites for electrical laminates, structural use in automotive components, aircraft and naval vessels. Thus, there exists a need for rigid silsesquioxane resins having increased flexural strength, flexural strain, fracture toughness, and fracture energy, without significant loss of modulus or degradation of thermal stability. In addition, rigid silsesquioxane resins have low dielectric constants and are useful as interlayer dielectric materials. Rigid silsesquioxane resins are also useful as abrasion resistant coatings. These applications require that the silsesquioxane resins exhibit high strength and toughness.
Conventional thermoset networks of high cross-link density, such as silsesquioxane resins, typically suffer from the drawback that when measures are taken to improve a mechanical property such as strength, fracture toughness, or modulus, one or more of the other properties suffers a detriment.
Various methods and compositions have been disclosed in the art for improving the mechanical properties of silicone resins including: 1) modifying the silicone resin with a rubber compound, as disclosed in U.S. Pat. No. 5,747,608 which describes a rubber-modified resin and U.S. Pat. No. 5,830,950 which describes a method of making the rubber-modified resin; 2) adding a silicone fluid to a silicone resin as disclosed in U.S. Pat. No. 5,034,061 wherein a silicone resin/fluid polymer is adapted to form a transparent, shatter-resistant coating.
While the above referenced patents offer improvements in the toughness of silicone resins, there is an additional need to further improve the toughness of silicone materials for use in high strength applications, such as those described above.
Therefore, it is an object of this invention to provide a process that may be utilized to prepare a cured silsesquioxane resin having high fracture toughness with minimal loss of modulus.
As described above, silsesquioxane copolymer resins can be formulated and cured into polymer products having very useful properties. However, the uncured copolymers are usually synthesized or produced having quite a broad range of molecular weights. In accordance with this invention, such a resin is fractionated into portions with smaller molecular weight ranges and each portion cured separately in the same desired formulation. Upon comparison of the separately cured samples it is found that a cured resin from a suitably selected molecular weight portion has selected superior properties as compared to the cured whole batch, e.g., a better combination of high fracture toughness and strength without loss of elastic modulus.
Thus, in accordance with a preferred embodiment of the invention, a process is provided for producing a cured silsesquioxane resin having high fracture toughness and strength without loss of elastic modulus comprising the steps of:
a) providing a silsesquioxane resin;
b) fractionating the silsesquioxane resin a plurality of times utilizing an organic solvent or using other approaches such as column fractionation or supercritical fluid extraction or dialysis methods or electrophoresis methods or any other method that separates polymeric mixtures according to molecular weight differences. When organic solvent is used for fractionation then step c follows:
c) stripping the silsesquioxane resin of step b) of excess solvent;
d) selecting an appropriate fraction;
e) mixing the silsesquioxane resin of step c) with a cross-linking compound to form a curable composition;
f) curing the curable composition to form a cured resin having a higher fracture toughness than that of the resin of step a).
This invention relates to a hydrosilylation reaction curable composition and process that is used to prepare a cured silsesquioxane resin. This curable composition comprises: (A) a silsesquioxane copolymer, (B) a silicon hydride containing hydrocarbon, silane or siloxane as a crosslinker, (C) a catalyst, (D) an optional inhibitor and (E) an optional solvent.
Component (A) is a silsesquioxane copolymer comprising units that have the empirical formula R1aR2bR3cSiO(4xe2x88x92axe2x88x92bxe2x88x92c)/2, wherein: a is zero or a positive number, b is zero or a positive number, c is zero or a positive number, with the provisos that 0.8xe2x89xa6(a+b+c)xe2x89xa63.0 and component (A) has an average of at least 2 R1 groups per molecule, and each R1 is independently selected from monovalent hydrocarbon groups having aliphatic unsaturation, and each R2 and each R3 are independently selected from monovalent hydrocarbon groups and hydrogen. Preferably, R1 is an alkenyl group such as vinyl or allyl. Typically, R2 and R3 are nonfunctional groups selected from the group consisting of alkyl and aryl groups. Suitable alkyl groups include methyl, ethyl, isopropyl, n-butyl, and isobutyl groups. Suitable aryl groups include phenyl groups. Suitable silsesquioxane copolymers for component (A) are exemplified by (PhSiO3/2)0.75(ViMe2SiO1/2)0.25, where Ph is a phenyl group, Vi represents a vinyl group, and Me represents a methyl group.
Component (B) is a silicon hydride containing hydrocarbon having the general formula HaR1bSiR2SiR1cHd where R1 is a monovalent hydrocarbon group and R2 is a divalent hydrocarbon group and where a and dxe2x89xa71, and a+b=c+d=3. The general formula HaR1bSiR2SiR1cHd although preferred in the present invention is not exclusive of other hydrido silyl compounds that can function as cross-linkers of the component (A). Specifically a formula such as the above, but where R2 is a trivalent hydrocarbon group can also be suitable as component (B). Other options for component (B) can be mixtures of hydrido-silyl compounds as well.
Suitable silicon hydride containing hydrocarbons of component (B) with the aforementioned formula can be prepared by a Grignard reaction process. For example, one method for making a silyl-terminated hydrocarbon for use in this invention includes heating to a temperature of room temperature to 200xc2x0 C., preferably 50xc2x0 C., a combination of magnesium and a solvent such as diethylether or tetrahydrofuran. A di-halogenated hydrocarbon, such as dibromobenzene is then added to the magnesium and solvent over a period of several hours.
After complete addition of the di-halogenated hydrocarbon, a halogenated silane, such as dimethylhydrogenchlorosilane, is then added, and an optional organic solvent can also be added. The resulting mixture is then heated for a period of several hours at a temperature of 50 to 65xc2x0 C. Any excess halogenated silane is then removed by any convenient means, such as neutralization with a saturated aqueous solution of NH4 Cl. The resulting product can then be dried with a drying agent such as magnesium sulfate and then purified by distillation.
An example of such a silicon hydride containing hydrocarbon produced by a Grignard reaction includes p-bis(dimethylsilyl)benzene which is commercially available from Gelest, Inc. of Tullytown, Pa.
Component (B) may also be a silane or siloxane that contains at least two silicon hydride functionalities that will cross-link with the vinyl group of component (A). Examples of suitable silanes and siloxanes that may be utilized as component (B) include diphenylsilane and hexamethyltrisiloxane. Such compounds are commercially available from Gelast, Inc. of Tullytown, Pa. and United Chemical Technologies of Bristol, Pa.
Components (A) and (B) are added to the composition in amounts such that the molar ratio of silicon bonded hydrogen atoms (SiH) to unsaturated groups (Cxe2x95x90C) (SiH:Cxe2x95x90C) ranges from 1.0:1.0 to 1.5:1.0. Preferably, the ratio is in the range of 1.1:1.0 to 1.5:1.0. If the ratio is less than 1.0:1.0, the properties of the cured silsesquioxane resin will be compromised because curing will be incomplete. The amounts of components (A) and (B) in the composition will depend on the number of Cxe2x95x90C and Sixe2x80x94H groups per molecule. However, the amount of component (A) is typically 50 to 98 weight % of the composition, and the amount of component (B) is typically 2 to 50 weight % of the composition.
Component (C) is a hydrosilylation reaction catalyst. Typically, component (C) is a platinum catalyst added to the composition in an amount sufficient to provide 1 to 10 ppm of platinum based on the weight of the composition. Component (C) is exemplified by platinum catalysts such as chloroplatinic acid, alcohol solutions of chloroplatinic acid, dichlorobis(triphenylphosphine)platinum(II), platinum chloride, platinum oxide, complexes of platinum compounds with unsaturated organic compounds such as olefins, complexes of platinum compounds with organosiloxanes containing unsaturated hydrocarbon groups, such as Karstedts catalyst (i.e. a complex of chloroplatinic acid with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane) and 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane, and complexes of platinum compounds with organosiloxanes, wherein the complexes are embedded in organosiloxane resins. A particularly preferred catalyst is a 0.5% platinum containing platinum-divinyltetramethyidisiloxane complex commercially available from Dow Corning Corporation, Midland, Mich.
Component (D) may include an optional catalyst inhibitor, typically added when a one part composition is prepared. Suitable inhibitors are disclosed in U.S. Pat. No. 3,445,420 to Kookootsedes et al., May 20, 1969, which is hereby incorporated by reference for the purpose of describing catalyst inhibitors. Component (D) is preferably an acetylenic alcohol such as methylbutynol or ethynyl cyclohexanol. Component (D) is more preferably ethynyl cyclohexanol. Other examples of inhibitors include diethyl maleate, diethyl fumamate, bis(2-methoxy-1-methylethyl)maleate, 1-ethynyl-1-cyclohexanol, 3,5-dimethyl-1-hexyn-3-ol, 2-phenyl-3-butyn-2-ol, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine, ethylenediamine, diphenylphosphine, diphenylphosphite, trioctylphosphine, diethylphenylphosphonite, and methyldiphenylphosphinite.
Component (D) is present at 0 to 0.05 weight % of the hydrosilylation reaction curable composition. Component (D) typically represents 0.0001 to 0.05 weight % of the curable composition. Component (D) preferably represents 0.0005 to 0.01 weight percent of the total amount of the curable composition. Component (D) more preferably represents 0.001 to 0.004 weight percent of the total amount of the curable composition.
Components (A), (B), (C) and (D) comprise 10 to 100 weight % of the composition. The composition may further comprise one or more optional components including a solvent (E).
The hydrosilylation reaction curable composition comprising components (A), (B), and (C), (D) and any optional components can be dissolved in component (E), an optional solvent. Typically, the amount of solvent is 0 to 90 weight %, preferably 0 to 50 weight % of the curable composition. The solvent can be an alcohol such as methyl, ethyl, isopropyl, and t-butyl alcohol; a ketone such as acetone, methylethyl ketone, and methyl isobutyl ketone; an aromatic hydrocarbon such as benzene, toluene, and xylene; an aliphatic hydrocarbon such as heptane, hexane, and octane; a glycol ether such as propylene glycol methyl ether, dipropylene glycol methyl ether, propylene glycol n-butyl ether, propylene glycol n-propyl ether, and ethylene glycol n-butyl ether; a halogenated hydrocarbon such as dichloromethane, 1,1,1-trichloroethane and methylene chloride; chloroform; dimethyl sulfoxide; dimethyl formarnide; acetonitrile and tetrahydrofuran. A preferred solvent is toluene.
The curable composition of the present invention is preferably a two part composition. The first part comprising component (A) is prepared by a fractionation technique that will be described in more detail below. The second part is prepared by mixing component (B) with component (C) and any optional compounds such as component (D) and component (E) and thereafter keeping the first and second parts separate. The first and second parts are mixed immediately before use.
The mixing of the curable composition of the present invention may also include the step of degassing the composition before curing. Degassing is typically carried out by subjecting the composition to a mild vacuum.
As referenced above the process of the present invention includes that component (A) is subjected to a fractionation prior to combing the fractions with other components. The silsesquioxane resin of component (A) is fractionated using an organic solvent in combination with an organic non-solvent (a solvent in which the silsequioxane resin is not soluble), preferably reagent grade methyl alcohol, although other organic solvents that are a poor solvent for silsesquioxane may be utilized by the present invention. The fractionation process is preferably performed utilizing a silsequioxane resin dispersed in a solvent such as toluene that is stirred as the fractionation organic solvent is added to the solution. The mixture of the silsesquioxane resin and organic solvent is allowed to separate until two clear phases are formed. The lower positioned or higher molecular weight fraction is removed leaving the upper phase. The upper phase may then be subjected to another fractionation using the same organic solvent and method outlined above.
The process of the present invention preferably includes fractionating component (A) three times utilizing the method described above, yielding four different phases. Each fraction can then be combined with components (B), (C), (D) and (E) as described above and subjected to various curing operations, as described in the examples section, to form a cured silsesquioxane resin. It must also be pointed out that recombination of selected fractions can become part of the process if one chooses to modulate the molecular weights and percent functionalities of component (A).